Many manufacturing processes require accurate and repeatable pressure measurements during critical process steps. These processes may rely on diaphragm pressure gauges to achieve an accurate determination of process chamber pressure. Diaphragm pressure gauges are widely used in the semiconductor industry. In part, this is because they are typically well suited to the corrosive services of this industry. They are also favored because of their high accuracy and immunity to contamination.
A typical diaphragm pressure gauge has a pressure port that is open to a process chamber or conduit containing a media being measured, a diaphragm or bellows exposed to the media being measured and a header assembly attached to the pressure port. The header assembly creates a chamber, referred to the vacuum reference chamber, which is typically (although not necessarily) evacuated to create a vacuum reference, on the opposite side of the diaphragm from the media being measured. The diaphragm or bellows is typically made of a flexible metal sheet and separates the vacuum reference chamber from the media being measured. According to some prior art sensor manufacturing techniques, the vacuum is formed on the reference side of the diaphragm by electron-beam welding the header assembly to the pressure port in a vacuum. Tungsten Inert Gas (“TIG”), laser or other welding techniques are typically employed to join the header assembly to the pressure port, with the chamber being evacuated at a later time. In addition, a getter material can be installed in the vacuum in order to maintain vacuum integrity over time.
Based on the pressure difference between the vacuum reference and the pressure of the media being measured, the diaphragm will flex. Piezoresistive strain gauges are attached to the diaphragm to detect the amount of flexion in the diaphragm. As the flexion in the diaphragm changes, the resistance of the strain gauges will change. The resistance changes of the strain gauges can be correlated to a particular pressure in the process chamber or conduit. In absolute pressures measuring devices, the strain gauges attached to the diaphragm are typically located in the vacuum reference chamber of the diaphragm vacuum gauge. Therefore, signals from the strain gauges must be transmitted out of the vacuum reference chamber to electronics for processing.
In some prior art systems, flexible cables of polyimide and copper/tin are used for this purpose. One end of a cable is attached to a portion of the pressure port using adhesives. A small wire connects the strain gauge to this end of the cable using wire bonding or soldering. The flexible cable is not generally attached directly to the strain gauge as doing so can put undue stress on the strain gauge. The other end of the flexible cable is soldered to pins on the header assembly. The pin passes through the header assembly (e.g., by means of a glass or ceramic feed-through) to transmit the signal from the cable to the printed circuit board (“PCB”) outside of the header assembly. Socket or pin-type interfaces are then used to transmit signals from the diaphragm pressure gauge's PCB to external electronics.
One disadvantage of prior art systems is that the adhesives and elastomers associated with cabling in the vacuum have a higher vapor pressure compared to metals and ceramics because of their tendency for gas evolution at relatively low pressures. In other words, when used inside a vacuum or low pressure chamber, the elastomers or adhesives become a source of outgassing. As the temperature increases, the outgassing becomes worse. This can result in an increased pressure on the vacuum side of the diaphragm, leading to an unstable device output signal (e.g., reduction in signal) over time. Additionally, outgassing in the vacuum reference can cause thermally-induced errors. When the temperature increases the pressure of the vacuum reference will increase, causing the transducer output signal to reduce). Another disadvantage of this system is that the cables can interfere with movement of the diaphragm, decreasing the accuracy and stability of the diaphragm pressure gauge. Yet another problem with this method is that the flexible cables connecting the strain gauges to pins in the housing can become disconnected. To correct this problem, the hermitic seal of the pressure gauge must be broken, the cabling fixed and the vacuum reestablished.